Abstract Position-specific isotope analysis has shown its potential to reveal information regarding formation, migration, and conversion procesess of hydrocarbons. The intramolecular isotope compositions of butane are promising to serve as a new thermometer and tracer. Therefore, position-specific isotope signatures in butane at equilibrium are needed for calibrating experimental measurements, establishing new geothermometers, and recognizing kinetic isotope effects. Here we conduct quantum chemistry modeling with corrections beyond the harmonic approximation and the Born-Oppenheimer approximation to obtain accurate intramolecular and intermolecular carbon and hydrogen isotope fractionation factors for butane isomers at equilibrium. Temperature dependences of these equilibrium isotope effects are presented for the range from 0 °C to 726.85 °C. The contribution of higher-order energy terms to 1000lnα values beyond the Bigeleisen-Mayer equation is found to be comparable to the magnitude of current experimental precisions. In addition to the significance of anharmonicity, the contribution of hindered internal rotation and diagonal Born-Oppenheimer correction is found to be important for accurate predictions of position-specific hydrogen isotope equilibriums. The abundance ratio of n-butane to i-butane at equilibriums is also calculated at various temperatures. Our results provide fundamental understanding of equilibrium properties for studying position-specific isotope effects in butane.

中文翻译：

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丁烷异构体中特定位置碳氢同位素平衡的理论计算

摘要 位置特异性同位素分析显示了其揭示有关碳氢化合物形成、迁移和转化过程信息的潜力。丁烷的分子内同位素组成有望用作新的温度计和示踪剂。因此，平衡时丁烷中特定位置的同位素特征对于校准实验测量、建立新的地温计和识别动力学同位素效应是必要的。在这里，我们使用谐波近似和 Born-Oppenheimer 近似以外的校正进行量子化学建模，以获得平衡时丁烷异构体的准确分子内和分子间碳和氢同位素分馏因子。这些平衡同位素效应的温度依赖性在 0 °C 到 726.85 °C 的范围内呈现。发现高阶能量项对超出 Bigeleisen-Mayer 方程的 1000lnα 值的贡献与当前实验精度的大小相当。除了非调和性的重要性之外，还发现受阻内旋和对角波恩-奥本海默校正的贡献对于准确预测特定位置的氢同位素平衡很重要。平衡时正丁烷与异丁烷的丰度比也在不同温度下计算。我们的结果为研究丁烷中特定位置的同位素效应提供了对平衡特性的基本理解。除了非调和性的重要性之外，还发现受阻内旋和对角波恩-奥本海默校正的贡献对于准确预测特定位置的氢同位素平衡很重要。平衡时正丁烷与异丁烷的丰度比也在不同温度下计算。我们的结果为研究丁烷中特定位置的同位素效应提供了对平衡特性的基本理解。除了非调和性的重要性之外，还发现受阻内旋和对角波恩-奥本海默校正的贡献对于准确预测特定位置的氢同位素平衡很重要。平衡时正丁烷与异丁烷的丰度比也在不同温度下计算。我们的结果为研究丁烷中特定位置的同位素效应提供了对平衡特性的基本理解。